In the operation of chain stitch type sewing machines thread from a single supply is projected by the sewing needle through the fabric or other material being sewed in the form of a small loop. A looper mechanism, underneath the material to be sewn, engages the loop projected by the needle, not only retaining the loop while the needle is retracted but also displacing the loop relative to the fabric, in the direction of sewing, so that the next loop projected through the fabric by the needle passes though the loop previously formed. In this manner, the successive chain of stitches is formed using a single, continuous thread.
Although chain stitching is fast and economical, one of the weaknesses of the chain stitching system is that an interruption anywhere in the chain can permit the entire series of stitching to be pulled out. Under high speed sewing conditions, most equipment will from time to time momentarily malfunction, such that a stitch loop may not properly engage with the loop formed by the preceding stitch. In such cases, the sewing equipment will simply continue through its normal sequence of operations, commencing a new series of stitches following the skipped stitch. The machine operator, typically, will be entirely unaware that a stitch has been skipped, and the skip is not readily evident in the fabric itself, except by a painstaking, stitch-by-stitch inspection, which of course is highly impracticable. Accordingly, the fabric segment with the undetected skipped stitch in it typically continues to go through its normal production sequence, in which it is incorporated into a finished product. Because of the friction present in the fabric itself, the weakened chain of stitching may hold for a considerable period of time, sufficient to enable the defective part to go through the entire production sequence and have considerable value added thereto and associated therewith. For example, the fabric having a defective sequence of chain stitching may become part of the upholstery of an automobile or an expensive piece of furniture. During service, the defective stitching will prematurely give away and unravel resulting in a possibly defective auto or article of furniture. Thus, by reason of the lack of detection of a single skipped stitch, a manufacturer may be exposed to a substantial expense involved in the replacement or repair of a much larger article.
Although a variety of detectors are available for sensing and actual breakage in a thread, a skipped stitch does not involve a break in the thread, bur merely a failure from time to time for the stitched loop to be picked up and properly engaged by the next successive loop. This may involve only a single stitch, after which the sewing sequence returns to normal.
In accordance with the present invention, a novel and advantageous arrangement is provided for sensing the fact that a single stitch has been skipped in a sewing sequence and executing a control operation response thereto. Particularly, a standard chain stitch type sewing machine is modified by providing, between the primary thread tension control device and the sewing needle, a yieldable element which bears on the thread and displaces the thread as a function of the tension therein. If a stitch is skipped, the typical result is a slight withdrawal of the thread with the retracting needle, instead of the usual holding or pulling down on the thread by the looper mechanism. Accordingly, the tension in the thread, which is otherwise rather constant, is momentarily reduced. This permits the yieldable element bearing on the thread to displace it slightly, and this in turn is detected by a suitable sensing device.
Most advantageously, the sensing device does not stop the sewing machine, but rather actuates a marking device which simply places a high visability marking on the fabric seam in the area of the defect. This can be readily observed later on during an inspection stage and the defect easily repaired. The marking device may be in the form of a one-shot pulse valve, actuated by the sensing device to provide a single actuating pulse such as a pulse of air under pressure. This may serve to actuate a cylinder-mounted inker for example, which descends onto the fabric and places an appropriate mark thereon.
In accordance with one of the more specific aspects of the invention, the yieldable element of the sensing device may advantageously comprise the torsion spring element of a conventional thread tensioning device, as used more typically in bobbin or lock-stitch sewing machines. In the case of the present invention, the last mentioned thread tensioning device, herein referred to as a secondary thread tension device, is not relied upon to establish the thread tension of the chain stitch equipment but serves primarily as a convenient and economical means to provide a yieldable element acting on the thread and responsive to its tension. A sensing device, such as a sensitive microswitch, for example, is positioned to sense the position of the torsion spring element of the secondary tension device. When there is a momentary loss of thread tension, resulting from a skipped stitch, the torsion spring momentarily deflects, and its movement is sensed to execute the desired control function.
For a more complete understanding of the invention, reference should be made to the following detailed description of a preferred embodiment, and to the accompanying drawing.